Document best practice setup for labelOpticDeriver

[fonghou]

Hi @nikita-volkov,

I found this part of README document is not sufficient for labelOpticDeriver.

domain/README.md

Line 525 in 062d559

declare (Just (False, True)) (stdDeriver <> labelOpticDeriver)

1. When the second flag in declare (Just (False, False)) is set to False, hasFieldDeriver in stdDeriver produces "Illegal instance declaration for GHC.Records.HasField ... already has a field ...", which labelOpticDeriver also uses. Removing hasFieldDeriver fixes the compile error.
2. stdDeriver includes constructorIsLabelDeriver, mapperIsLabelDeriver, accessorIsLabelDeriver, though they don't produce any compile error they seem reductant to optics review, over, set operators.
3. Documentation only mentioned DuplicateRecordFields (typoed as DuplicateRecords somewhere). I found that, in order to use non-prefix record fields declare (Just (False, False)), it's best to enable these GHC extentions:

•   DuplicateRecordFields (always)
  

•   NoFieldSelectors (always GHC 9.2+), and optionally OverloadedRecordDot (caveat partial HasField for sum types)
  

•   NamedFieldPuns, and/or RecordWildCards (ie. concise field names in pattern match but no record accessor functions)
  

Just want to share them first, and discuss where the best of places to put them (documentation or TemplateHaskell codegen).

Cheers!

[nikita-volkov]

Thanks! I agree, the docs need improvement. I think it would be best to cover such details in the Haddocks of the according derivers.

Not sure what you meant by placing the docs in the TemplateHaskell codegen.

[nikita-volkov]

It's worth mentioning that OverloadedRecordDot was not a thing when the docs here were written, so the compatibility with it was not mentioned. It'd be great to have the instructions on how to use "domain" with OverloadedRecordDot covered.

[fonghou]

Thanks! I agree, the docs need improvement. I think it would be best to cover such details in the Haddocks of the according derivers.

Not sure what you meant by placing the docs in the TemplateHaskell codegen.

I meant define/export another top level stdDeriver without hasField/isLabelDeriver in DomainOptics module (name it stdOpticsDeriver?). Currently it must be exported from a separate module in project just to make TemplateHaskell stages happy. Yes, I agree Haddock is the best place, maybe put all of above under stdOpticsDeriver.

[fonghou]

It's worth mentioning that OverloadedRecordDot was not a thing when the docs here were written, so the compatibility with it was not mentioned. It'd be great to have the instructions on how to use "domain" with OverloadedRecordDot covered.

Understood. I'm actually on the fence about it. It works with the setting I gave above. Pros is it looks nicer (no %^~# chars :-) if all you have are records. Cons is that it can't handle Affine fields for sum types (ie. runtime errors).